<!doctype html>
<html>
<head>
<meta charset="utf-8">
<script> 
window.onload = function(){ 
	    var box = document.getElementById("box");
	    box.style.width = "400px";
	    box.style.height = "200px";
}
</script>
</head>
<body>
<div id="box" style="border:solid 1px red" >盒子</div>
</body>
</html> 
